1. Field of the Invention
The present invention relates to a motor control circuit for a wire bonding apparatus.
2. Prior Art
FIGS. 4(a) through 4(g) illustrate one method of wire bonding for semiconductor devices, etc., and FIG. 5 shows a wire bonding apparatus for such a method.
As shown in FIG. 5, a bonding head 11 is mounted on an XY table 10 which is driven by an X-axis motor and a Y-axis motor (both not shown). A supporting shaft 12 is mounted on the bonding head 11 in a rotatable fashion, and a horn holder 13 is fastened to this supporting shaft 12. A horn 14 is attached to the horn holder 13, and a capillary 4 (which is shown in FIGS. 4(a) to 4(g)) is provided at the tip end of the horn 14. The tip of a bonding wire 3 which is wound on a spool (not shown) is passed through the capillary 4.
A Z-axis motor 15 which is capable of making forward and reverse rotations is mounted on the bonding head 11, and a cam shaft 16 is coupled to the output shaft of the Z-axis motor 15. A linear cam or eccentric cam 17 is fastened to the cam shaft 16.
The horn holder 13 is urged by a spring 19 so that a cam follower 18 that is a roller rotatable on the horn holder 13 is pressed against the cam 17.
A detection plate 20 is attached to the undersurface of the horn holder 13. The detection plate 20 is on the opposite side of the supporting shaft 12 from the capillary and projects beyond the side surface of the horn holder 13. On the other hand, a linear sensor 21 is attached to the bonding head 11 so as to face the upper surface of the detection plate 20.
The reference numeral 22 is a linear motor which is used to set the bonding load. The magnet side 23 of the linear motor 22 is fastened to the bonding head 11, and the coil side 24 of the linear motor 22 is fastened to the horn holder 13.
The wire bonding method illustrated in FIGS. 4(a) through 4(g) is executed in the following manner:
First, as shown in FIG. 4(a), a ball 3a is formed by a spark discharge (created by an electric torch 5) on the wire 3 extending from the lower-end of the capillary 4. Then, the electric torch 5 moves in the direction indicated by the arrow.
Next, the XY table 10 (in FIG. 5) is driven so that the capillary 4 moves to a position above the first bonding point 1a as shown in FIG. 4(b). Then, the cam 17 is rotated in the forward direction as indicated by arrow A (in FIG. 5) so that the dropping profile of the cam 17 causes the horn holder 13 to pivot about the supporting shaft 12 as indicted by arrow B. As a result, as shown in FIG. 4(c), the capillary 4 is lowered, and the ball 3a of the wire 3 is connected to the first bonding point 1a.
Afterward, the cam 17 is rotated in the reverse direction so that the rising profile of the cam 17 causes the capillary 4 to be raised as shown in FIG. 4(d).
Next, the XY table 10 is driven so that the capillary 4 moves to a position above the second bonding point 2a as shown in FIG. 4(e). Then, the cam 17 is caused to rotate in the forward direction so that the capillary 4 is lowered and the wire 3 is connected to the second bonding point 2a. This is shown in FIG. 4(f).
Then, the cam 17 is rotated in the reverse direction, and the capillary 4 is raised to a predetermined position. Then, the clamper 6 is closed, and the capillary 4 and clamper 6 are raised together so that the wire 3 is cut as shown in FIG. 4(g).
One wire connection is thus completed.
Conventionally, the control of the Z-axis motor which drives the capillary 4 up and down is accomplished by a Z-axis control circuit such as that shown in FIG. 3.
In this diagram, a pulse input a (which is an operating command) is integrated by an integrator 30 to form an operating command pulse signal b; and a feedback speed signal c, a feedback compulsive pulse signal d, and a feedback pulse-by-pulse signal e are synthesized with the pulse signal b, thus obtaining a synthesized signal f.
The synthesized signal f is inputted into an amplifier 31, and motor gain control is accomplished with a fixed gain 32, and then the Z-axis motor 15 is operated via a motor driver 33.
In the above, the feedback speed signal c is obtained as follows: the output signal of an encoder 34 installed on the Z-axis motor 15 is waveform-shaped by a waveform shaping circuit 35 to produce a waveform-shaped signal g (Sin or Cos wave), which is then converted into a pulse signal h (actual operating pulse signal) by a pulse converter 36, and this pulse signal h is integrated by an integrator 37 so as to become the feedback speed signal c. This signal c performs speed feedback.
On the other hand, the feedback cumulative pulse signal d is a signal which feeds back the result (cumulative pulse) obtained in a comparison of the operating command pulse input a with the actual operating pulse signal h. The comparison is performed by a comparator 38.
Lastly, the feedback pulse-by-pulse signal e performs feedback within a single pulse. This signal is obtained as follows: the waveform-shaped signal g (Sin or Cos wave) which has been waveform-shaped by the waveform shaping circuit 35 is further converted by an F/V circuit (not shown) to form a sawtooth wave signal i, which is further differentiated by a differentiation circuit 39.
In the wire bonding operation shown in FIGS. 4(a) through 4(g), the capillary lowering operations in FIGS. 4(b) and 4(c) and FIGS. 4(e) and 4(f) are performed as shown in FIGS. 2(a) and 2(b) in order to increase the productivity. FIG. 2(a) shows the rotational speed of the Z-axis motor 15, and FIG. 2(b) shows the lowering path of the capillary 4.
More specifically, the Z-axis motor 15 is first caused to rotate in high-speed at 50, so that the capillary 4 is lowered rapidly at 55. Afterward, the rotational speed of the Z-axis motor 15 is reduced at 51, thus the lowering speed of the capillary 4 is reduced. Next, the Z-axis motor 15 is caused to rotate in a constant low-speed operation at 52, so that the capillary 4 is lowered at a low speed at 57 and then touches a workpiece at 53, thus letting the ball 3a or wire 3 contact the first bonding point la or the second bonding point 2a.
FIG. 2(c) shows the damper load 70 and the bonding load 71. The damper load 70 is for a stabilization of the capillary 4 during its raising and lowering actions, and the bonding load 71 is used during bonding. The damper load 70 and bonding load 71 are applied by the linear motor 22 that is shown in FIG. 5.
More specifically, until the Z-axis motor 15 enters the low-speed operation 52, a certain constant current is applied to the capillary 4. When the constant current is applied to the linear motor 22, the coil 24 of the motor 22 is repelled by the magnet 23 so that the horn holder 13 is pushed up, thus causing the cam follower 18 to be pressed against the cam 17. As a result, a large damper load 70 is applied to the capillary 4.
When the Z-axis motor 15 enters the low-speed operation 52, a current which is smaller than the above-described current is applied to the linear motor 22 so that a load (the bonding load 71) which is smaller than the damper load 70 is applied to the capillary 4. As a result, the ball 3a or the wire is bonded to the first bonding point 1a or the second bonding point 2a under a small bonding load 71.
FIG. 2(d) shows the ideal output waveform of the encoder 34 (see FIG. 3) in a case where the Z-axis motor 15 rotates smoothly according to the Z-axis command speed shown in FIG. 2(a).
As seen from above, the control of the capillary 4 during the wire bonding operation involves a speed change-over operation (high speed to low speed, and low speed to high speed) and a vertical movement change-over (raising to lowering, and lowering to raising). In the above prior art, the same feedback control is performed by the fixed gain circuit 32 shown in FIG. 3, regardless of which change-over operation is undergoing.
As to the change in the load on the Z-axis motor 15, the load applied to the cam 17 shown in FIG. 5 is released (i.e., the load is decreased) at the time of change-over from the large damper load 70 to the small bonding load 71 (as shown in FIG. 2(c). This occurs while the capillary 4 is being lowered, so that the cam 17 is in its dropping profile. Accordingly, during the application of the large damper load 70, the force which causes the cam 17 and Z-axis motor 15 to rotate in the direction of rotation of the cam 17 and Z-axis motor 15 is large.
However, when the load is switched from the large damper load 70 to the small bonding load 71, the force applied in the direction which causes the cam 17 and Z-axis motor 15 to rotate becomes weaker, and the rotation of the Z-axis motor 15 lags behind. As a result, immediately after such an change-over from the damper load 70 to the bonding load 71, a delayed output waveform 75 is outputted by the encoder 34 as shown in FIG. 2(e).
Accordingly, the feedback accomplished by the fixed gain circuit 32 acts to make the output waveform of FIG. 2(e) to be closer to the ideal operating state waveform which is shown in FIG. 2(d). However, feedback which works well during a high-speed operation is too strong for a low-speed operation. Accordingly, the load on the Z-axis motor 15 fluctuates, and in the case of a subsequent shift to low-speed operation, the actual operation lags behind the ideal operation shown in FIG. 2(d).
An attempt is made to catch up by causing the Z-axis motor 15 to rotate more rapidly. In this case, however, the rotational speed is increased by an excessive amount, and the operation advances too far. When the operation advances too far, feedback is applied which conversely slows down the rotation of the Z-axis motor. In this case, however, the rotational speed is excessively reduced, and the operation now lags behind again. Thus, as a result of the excessively strong application of feedback, an output waveform 76 is obtained which shows lags and advances instead of getting closer to the ideal operating state of FIG. 2(d). Accordingly, the rotational speed of the Z-axis motor 15 at the moment when the capillary comes into contact with the workpiece tends to vary with each ascending motion, so that there is a variation in the impact load at the time of the capillary's contact with the workpiece.
In the prior art described above, the same feedback is performed regardless of the operating state. Accordingly, when the load on the Z-axis motor 15 fluctuates, or when there is an abrupt change-over from high speed to low speed, etc., the Z-axis motor 15 does not rotate smoothly, and the resulting bonding stability is poor.
As a result, unnecessary vibration is generated, and the speed of the capillary at the time of contact with the semiconductor pellets or lead surfaces varies from bonding point to bonding point. Thus, there are differences in the impact load, which leads to a variation in the bonding strength and size of the pressure-bonded part of the wire and further to a variation in the shape of the wire loop.
In order to prevent this, it is necessary to take time to allow the Z-axis motor 15 to stabilize in the rotational speed, and then bring down the capillary after the stabilization of the rotation. In such cases, however, the operation speed of the apparatus is greatly slowed down.